The global carbon cycle is heavily influenced by the activities of man. For example, the combustion of fuels by man is believed to have resulted in a large increase in the amount of carbon dioxide present in the atmosphere. In the last hundred years, global fossil carbon emissions have increased by more than a factor of ten. As nations around the globe continue to become more industrialized, demands for energy are expected to increase dramatically. As such, in the absence of new technological solutions, it is believed that the trend toward increased fossil carbon emissions will continue.
Carbon dioxide is considered to be a “greenhouse” gas and is believed to have contributed to global warming trends. Carbon dioxide, along with water vapor, methane, nitrous oxide, and ozone, causes more heat to be retained by the Earth than would otherwise be captured. The global average air temperature near the Earth's surface rose 0.74±0.18° C. during the last 100 years. It is believed that this is due, at least in part, to the observed increase in greenhouse gas concentrations. Further increases in global temperatures may lead to various catastrophic effects including a rising sea level, increased extreme weather events, reduced agricultural yields, glacier retreat, and species extinction, amongst others.
In an effort to prevent catastrophic events from occurring, significant resources have been devoted to developing systems to reduce the amount of carbon dioxide emitted into the atmosphere. The various strategies pursued can be grouped into two broad categories: reduction of carbon emissions and capture of atmospheric carbon.
In nature, plants efficiently capture atmospheric carbon through the process of photosynthesis. Using sunlight as energy, plants convert carbon dioxide and water into the precursors of carbohydrates and other plant constituents. Many different types of plants and microorganisms capture considerable amounts of carbon dioxide. Algae are photosynthetic organisms that occur in most habitats. They vary from small, single-celled forms to complex multicellular forms. Algae are estimated to generate as much as 80 percent of the Earth's oxygen. It is also estimated that algae fix 90 gigatons of carbon per year.
Various attempts have been made at designing algae culture systems in order to capture carbon dioxide. In general, there are two types of algae culture systems: open culture systems and closed culture systems. Open culture systems are open to the atmosphere. They have the advantage of being relatively inexpensive to construct. However, open culture systems are subject to atmospheric temperature fluctuations, are susceptible to contamination issues, and suffer substantial losses of water due to evaporation. In contrast, closed culture systems are closed to the atmosphere and therefore provide the advantages of a controlled environment, lower evaporative water loss, and fewer contamination issues. However, many closed culture systems require relatively complex structures and therefore have substantially higher construction and operating costs. In addition, many closed culture systems have issues associated with insufficient light penetration, algae growth on walls that can be difficult to clean, and poor temperature control.
For at least these reasons, a need remains for algae cultivation systems and methods.